1. Field of the Invention
The present invention relates generally to the field of chemical vapor deposition and more specifically to an exhaust conductance system for a CVD fabrication reactor.
2. Description of the Related Art
Chemical vapor deposition relates generally to the process of passing a reactant gas over one or more substrates in order to deposit various materials thereon. This technique is commonly employed in the electronic arts as part of the process for fabricating semiconductor devices.
In the context of semiconductor fabrication, the substrate typically is a wafer approximately 50 to 300 millimeter in diameter. A wafer handler places one or more wafers in a reaction chamber through a gate valve, which is then closed. A reactant gas, which contains particles and particle-generating compounds to be deposited onto the wafers, is introduced into the reaction chamber through a separate passage. As the reactant gas passes over the wafers, the particular chemical compounds in the gas adhere to the surface of the wafers as a result of a thermal reaction or decomposition of the various gaseous materials.
After passing through the reaction chamber, the reactant gas exits the chamber through an exhaust conductance path. The exhaust conductance path typically leads to a scrubber or other device that prepares the reactant gas for proper disposal. As the reactant gas travels down the exhaust conductance path towards the scrubber, the chemical compounds in the reactant gas adhere to the walls of the conductance path, thus contaminating the system.
Upon completion of the deposition process, a purging gas is introduced into the reaction chamber at one or more sites in order to expel the reactant gas from the chamber. Typical purging gases are hydrogen, nitrogen, helium, or argon. Like the reactant gas, the purging gas travels through the reaction chamber and exits through the exhaust conductance path to the scrubber. Unlike the reactant gas, however, the purging gas does not deposit chemical compounds on the walls of the exhaust conductance path and, thus, does not contaminate the system.
After the reaction chamber has been purged, the gate valve is opened, and the deposited wafers are removed and replaced with undeposited wafers. The gate valve is then closed, and a new cycle of the chemical vapor deposition process commences.
In order to obtain commercially viable wafers, it is critical to control the thickness of the deposition layer. Ideally, the chemical compounds in the reactant gas would be deposited in a uniform thickness across the entire surface of each wafer. Several factors work against achieving this desired result, however. For example, as the reactant gas deposits onto the wafers, the concentration of the material to be deposited changes, resulting in a thinner deposition layer further down the reaction chamber. Similarly, variances in temperature at different locations within the reaction chamber cause the reactant gas to deposit onto the wafers at different rates. Furthermore, contamination within the reaction chamber leads to the adherence of unwanted particles to the surface of the wafers.
Improvements have been made in the field of chemical vapor deposition to correct the problems associated with variances in the concentration of chemical compounds in the reactant gas and variances in temperature within the chamber. Few improvements have been made, however, that effectively overcome the problems associated with contamination.
Contamination of the wafers may occur, for instance, if sudden changes in pressure within the semiconductor fabrication reactor cause particles to shear loose from the inner surface of the exhaust conductance path. These pressure pulses may occur when the hermetically sealed valve separating the reaction chamber from the wafer handler is opened, or when the reactor is activated or deactivated. Particles adhering to the inner surface of the exhaust conductance path also may shear loose under the force of the venting gas passing through the exhaust conductance path. Furthermore, vibrations from the reaction chamber or some external force may cause the particles to break loose. The detached particles then may travel upstream back towards the reaction chamber, via viscous flow, and ultimately contaminate the wafers.
Thus, there is a need in the field of chemical vapor deposition and semiconductor fabrication reactors for an improvement to correct contamination of wafers caused by particles adhering to the inner surface of traditional exhaust conductance paths.
The present invention is directed towards an improved exhaust conductance system for a semiconductor fabrication reactor that reduces the extent of contamination of wafers processed therein. A preferred embodiment of the exhaust conductance system of the present invention comprises two separate exhaust conductance paths selectable by way of a three-way valve. One of the exhaust conductance paths vents reactant gas (or any gas that adheres to the inner surface of the conductance path). The other exhaust conductance path vents purging gas (or any noncontaminating gas). The three-way valve allows for selection of the reactant gas exhaust conductance path during the processing and etching phases of the deposition cycle and selection of the purging gas exhaust conductance path during most other phases of the cycle. Because the latter, cleaner path can be selected during those phases of the cycle when the system is susceptible to pressure pulses, gas flow forces, or other forces likely to shear loose any particles adhering to the inner surface of the exhaust conductance path, there is less likelihood that particles will flow back into the reaction chamber and contaminate processed wafers or chamber walls.
The scope of the present invention is not limited to two exhaust conductance paths connected to the reaction chamber by a three-way valve. The present invention contemplates multiple exhaust conductance paths joined to the reaction chamber by appropriate valving. Use of more than one exhaust conductance path allows for the selection of separate paths for passage of contaminating and noncontaminating gases. Additional conductance paths may be included to provide for more specific separation of the gases.
The scope of the present invention also includes a method of using the improved exhaust conductance system. During the phases of the chemical vapor deposition cycle in which particles may form on the walls of the exhaust line the valve in the exhaust line is positioned so that the reaction chamber is open to the first or xe2x80x9cdirtyxe2x80x9d exhaust conductance path, which is connected to the scrubber. Typically, this would include process, etch, and bake processing steps. Upon completion of the processing phase, the three-way valve is positioned so that the reaction chamber is open to the second or xe2x80x9ccleanxe2x80x9d exhaust conductance path, and the xe2x80x9cdirtyxe2x80x9d exhaust conductance path is closed. Purging gas is then introduced into the reaction chamber and exits through the clean exhaust path. After purging, the interior of the reaction chamber is accessed, and the deposited wafers are replaced with undeposited wafers. The path in the clean line is also left open during idle or any wafer cool down step.
Further features and advantages of the present invention will become apparent to those of skill in the art in view of the detailed description of preferred embodiments, which follows, when considered together with the attached drawings and claims.